The main function of the random access of the LTE system is to acquire uplink synchronization and to allocate one cell unique identifier “Cell Radio Network Temporary Identifier (C-RNTI)” to the user equipment (UE) when establishing an initial network access, for example, transiting from the Radio Resource Control (RRC)_idle (IDLE) state to RRC_connected (CONNECTED) state. The random access process of the LTE system has the following two manners:
the first is non-contention based access manner. As to the non-contention based random access process, the eNodeB (eNB) can avoid contention by allocating one specific preamble sequence to the UE.
The second is contention based access manner. As to the contention based random access process, one preamble sequence randomly selected by the UE may cause a plurality of UEs to simultaneously select the same preamble sequence for transmission, resulting in collision, and then one contention resolution process is needed for performing processing, as shown in FIG. 1. First, the UE randomly selects one from the contention based random access preamble and initiates random access. After having received the preamble from the UE successfully, the eNB scuds one random access response to the UE, which includes the ID of the preamble. C-RNTI, information about uplink timing synchronization, and information about the radio resources allocated to the Msg3 and so on. In this case, the Msg3 is the first scheduled transmission (also referred to as Message 3, abbreviated as Msg3) sent over the uplink channel resource. Then, the UE sends the Msg3 to the eNB according to the radio resource information about the Msg3 in the received random access response, and the Msg3 includes a RRC message, UE ID, etc. At last, the eNB sends a random access contention resolution message Msg4 including UE ID, and the UE deems that the contention is resolved when the UE ID in the received Msg4 matches the UE ID sent in the Msg3.
According to the above random access manner, 64 preambles in one cell can be classified as two categories: contention based random access preamble and non-contention based random access preamble. However, the contention based random access preamble can also be divided into two groups: Group A and Group B, as shown in FIG. 2. At most times, the eNB has no idea about the reason for triggering random access nor about the cache state of the UE, therefore, after the random access preamble is received successfully, the eNB can only allocate one default standard resource configuration to the first transmission of the physical uplink shared channel (PUSCH), i.e. Msg3. However, the UE itself knows the reason for triggering random access and the size of the Msg3 and can measure the path loss of the radio channel, and according to two conditions of Msg3 size and Path loss, the UE can select a contention based random access preamble Group, wherein Group A corresponds to small Msg3 size and Group B corresponds to large Msg3 size. After having received the preamble sent by the UE successfully, the eNB can know the Msg3 size according to the Group where the preamble is located. Then, resources can be allocated to the Msg3 in the random access response according to the information of Msg3 size, thus the PUSCH resources of the Msg3 can be allocated with high efficiency.
Usually, 1 resource block is allocated to the Msg3 of Group AUE, and 2 RB resources are allocated to the Msg3 of Group BUE, however, it is not excluded here that more than 2 RD resources are allocated to the Msg3 of UE in the Group B according to different Msg3 sizes. However, such allocation of resources is not based on each subframe but allocated by the eNB semi-statically.